Field of the Invention
The present invention relates to a joint driving apparatus provided with a second link pivotally supported on a first link and adapted to control relative orientation of the first and second links as well as to a robot apparatus equipped with the joint driving apparatus.
Description of the Related Art
In recent years, articulated robot manipulators have been finding increasingly wider uses and expanding their application to areas, such as collaborative work with humans and assembly work in a factory, in which robots are required to perform flexible movements. A stable and wide-band force control function is required of such a robot apparatus to follow external forces. Thus, there is demand to construct a control system based on joint-level torque control (torque servo) instead of a widely used conventional motion control system based on joint position control (position servo). The torque control requires a torque detection unit capable of accurately detecting torque which is output from joints of a robot arm.
As this type of robot apparatus, for example, a multi-joint robot is known which is made up of plural links coupled by plural joint drive shafts. Such a multi-joint robot arm is configured such that forces acting on a member on an end side accumulate and act on a base member (base or mount). In order to improve responsiveness and mobility of the robot, it is necessary to make the entire robot compact and lightweight. For that, it is necessary to reduce the size and weight of a drive mechanism.
Conventionally, regarding this type of joint driving apparatus, a configuration is known in which a torque detection apparatus adapted to measure output torque of the joint via a bearing adapted to rotatably support an output link is installed between an output shaft (final stage) of the joint driving apparatus, including a motor and a reduction mechanism, and a drive shaft of an output link. Generally, this type of torque detection apparatus is made up of a strain gage or an elastic member adapted to deform according to applied torque and a sensor device adapted to detect a deformation amount or distortion amount of the elastic member, for example, optically or magnetically.
For example, a robot arm equipped with a torque sensor is disclosed in “A. Albu-Schaeffer, S. Haddadin, Ch. Ott, A. Stemmer, T. Wimboeck, G. Hirzinger, “The DLR lightweight robot: design and control concepts for robots in human environments”, Industrial Robot: An International Journal, Vol. 34, Iss: 5, pp. 376-385 (2007)” (hereinafter, referred to as an article by A. Albu-Schaeffer et al.). The robot arm disclosed in the article by A. Albu-Schaeffer et al. includes an inner ring coupled to a shaft driven by a servomotor via strain wave gearing, an outer ring coupled to a first member of the robot arm, and a torque sensor adapted to measure torque, i.e., rotational torque between the inner ring and outer ring. With the robot arm disclosed in the article by A. Albu-Schaeffer et al., relative displacement produced between the inner ring and outer ring when torque acts around a rotating shaft of a first member is detected as distortion of the elastic member of the torque sensor, thereby measuring the rotational torque acting on the first member.
However, in the configuration described in the article by A. Albu-Schaeffer et al., the torque sensor rotates together with a measuring object, causing, for example, a sensor cable attached to the torque sensor to deform along with motion of the drive shaft. Consequently, there is a problem in that when a reaction force from the sensor cable cannot be ignored, such as when a robot joint has a large operation angle, it is difficult to detect torque accurately. Also, with the configuration in the article by A. Albu-Schaeffer et al., a cable routing mechanism tends to become complicated, and it is not easy to ensure durability of the sensor cable.
Also, as a torque detection apparatus for a drive system of a sphere tire adapted to move a robot apparatus, a configuration is proposed, for example, in Japanese Patent Application Laid-Open No. 2012-047460. The configuration disclosed in Japanese Patent Application Laid-Open No. 2012-047460 includes a driving unit made up of a rotor and a stator, where the rotor has a spindle in a first axial direction and the stator causes the rotor to rotate around the spindle. The torque detection apparatus includes a strain body and a detection element, where the strain body is placed concentrically with the rotor and provided with a first end portion fixed to a base portion and a second end portion fixed to the stator while the detection element is attached to the strain body and adapted to detect distortion of the strain body around a first axis.
However, in the configuration described in Japanese Patent Application Laid-Open No. 2012-047460, the strain body (elastic member) of the torque detection apparatus is placed between the base portion and a support mechanism (bearing) for the driving mechanism. With this configuration, a force acting on a driving torque output link acts directly on the strain body, posing a problem in that a torque sensor is susceptible to cross-axis components when the configuration is applied to a joint mechanism of the robot apparatus. Consequently, sensor output values may fluctuate under the influence of disturbance forces other than the desired torque around the drive shaft, which could result in a failure to detect output torque accurately (hereinafter, this problem will be referred to as interfering with the other axis or crosstalk).
Japanese Patent Application Laid-Open No. 2012-047460 presents a configuration in which the driving mechanism is pivotally supported by the support mechanism made up of the bearing and a frame body to reduce the influence of cross-axis forces acting on the elastic member of the torque sensor such as described above. However, this configuration will increase in complexity and size because of the support mechanism which supports the driving mechanism. In particular, when one attempts to apply the configuration of Japanese Patent Application Laid-Open No. 2012-047460 to a joint of the robot apparatus, rigidity and strength equal to or higher than those of links joined via the joint have to be secured for the bearing of the support mechanism for the driving mechanism in order to maintain rigidity and strength of the entire joint. Thus, as a joint driving apparatus of the robot apparatus, a joint area may become too large in mass and size.